1. Field of the Invention
The present invention relates to sub-ranging analog-to-digital (A/D) converters that convert an analog signal to a multi-bit digital signal in series of stages, with each stage determining a less significant bit or bits of the digital signal than the preceding stage and at least one stage determining more than one bit of the digital signal.
2. Description of the Related Art
A digital signal is a signal that must be in one of two stages or values for a defined period time, or transitioning therebetween. Typically, the two stages are referred to as a logical "0" state and a logical "1" state. Each digital signal is commonly referred to as a bit and several digital signals can be combined to produce a multi-bit digital signal with each bit having a different significance. For instance, the number seven may be represented by the 4-bit digital signal "0111" in which each of the bits from right to left represents an increasing power of 2 beginning with 2.sup.0 and the value of the bit, logical "0" or logical "1", indicating whether the associated power of 2 is used in representing the number 7. Consequently, the digital signal "0111" can be translated as (0.times.2.sup.3)+(1.times.2.sup.2)+(1.times.2.sup.1)+(1.times.2.sup.0)=7. In contrast, an analog signal is a signal that is capable of continuous variation. For example, a signal representative of air temperature over twenty-four hour period may vary continuously over a range of values during the twenty-four hour period.
Many applications require an analog signal to be converted into a representative or corresponding digital signal so that a digital computer, which is incapable of directly processing the analog signal, can process the information embodied in the analog signal. For instance, in many temperature control systems, an analog signal that is representative of the temperature at some point in the system must be converted into a digital signal so that at a digital controller can process the information in the analog signal and make the appropriate adjustments to the system. For example, the digital controller may cause the temperature at the aforementioned point in the system to be raised or lowered. To address this need, analog-to-digital (A/D) converters have been developed.
Analog-to-digital converters take an applied analog signal and convert it to a representative digital signal comprised of at least one bit, but more typically several bits, with each bit having either a logical "0" or a logical "1" value. For example, an analog signal of fourteen volts may be represented by the 4-bit digital signal "1110" and an analog signal of 7 volts may be represented by the digital signal "0111."
Several types of A/D converters are presently in existence. One type of A/D converter, known as a flash converter, determines the values or states of all the bits in the digital signal that is representative of an applied analog signal in one stage. Consequently, the values of all the bits are determined at substantially the same time. The flash converter operates by comparing the applied analog signal to a series of reference signals that are representative of all of the multi-bit digital signals in a defined range and then uses the result of the comparison to determine the value of the bits in the corresponding multi-bit digital signal. For instance, in a 4-bit flash converter, the analog signal is simultaneously compared to fifteen reference signals, with the fifteen reference signals dividing the input range into sixteen possible 4-bit combinations. The results of the fifteen comparisons are used to determine the 4-bit digital signal representative of the applied analog signal.
Another type of A/D converter known as a successive approximation A/D converter, uses a series of successive stages to determine the digital signal representative of an applied analog signal, with each stage determining only one bit of the digital signal. Stated another way, the successive approximation A/D converter determines the digital signal representative of an applied analog signal one bit at a time.
Yet a further type of A/D converter, known as a sub-ranging A/D converter, combines features of both the flash and successive approximation A/D converters. More specifically, the sub-ranging A/D converter utilizes a plurality of stages, like a successive approximation A/D converter, to determine the values of the multi-bit digital signal that is representative of the applied analog signal, with the initial stage determining the value of the most significant bit or bits in the multi-bit digital signal and the succeeding stages determining the values of progressively less significant bits of the multi-bit digital signal. Further, at least one of the stages of the sub-ranging A/D converter determines the values of a plurality of the bits of the digital signal at substantially the same time, just as with the flash A/D converter.
While the sub-ranging A/D converter provides a viable compromise between the high speed of flash A/D converters and the low speed of successive approximation A/D converters, there is still a desire to increase the speed of sub-ranging A/D converters. However, to increase conversion speed, presently known sub-ranging A/D converters require a substantial increase in power. The increase in power needed to achieve a corresponding increase in speed, in many applications, is undesirable. In addition to the desire to increase the conversion speed of sub-ranging A/D converters while avoiding substantial increases in power consumption, there is also a desire to simply conserve or reduce power consumption of sub-ranging A/D converters.